Why are ballasted bridges and trestles replacing open deck structures?
dd
I’m sure that the Mudchicken knows more about this than me but I’ll try anyway. I would think that a ballasted deck would result in a more stable rail structure which would stay in line and in gauge better.
The problem comes when the track on either side of the bridge needs to be ballasted and, thus, raised. Since the track on the bridge can’t be raised (not practically, anyhow), a transition needs to be made on each end so there won’t be a sudden dip.
Old Timer
Every time you go to surface the track, an open deck bridge is a maintenance nightmare. It’s fixed, the regular ballast section is not. The surfacing machinery must start on the bridge and work its way out. You wind up with permanent “dips” in an otherwise permanently smooth track surface and, as mentioned above, the track modulus (support stiffness) changes drastically between the two (dynamic structure to static structure)…
UP’s main transcon route is full of open deck bridges. The BNSF transcon (Santa Fe’s) is almost void of open deck bridges except for some VERY long older E-65 steel bridges. (UP’s replacement of the Kate Shelly bridges are ballast deck for a reason.)
Ballasted bridges generate a lot less noise than open decked ones due to the vibration attenuation by the ballast. Could be part of the reason
While we are at it…You just do NOT abitrarilly add a balast deck to an open deck bridge. All that extra dead weight just lessens the load carrying capacity of the bridge.
When we start talking about new “long span” bridges being open decked, we are talking about lengths greater than 250 feet. (and being Cooper’s E-80 or better)
Since this came up, let me ask this, yesterday I was walking along the old PRR mainline (chicago-Pittsburgh line) And came upon a bridge that at one time was dual track, but had been singled by conrail a while back.
The bridge was a riveted steel plate bridge, with numerous riveted steel box beams underneath for support
The surviving track was ballasted all the way across the bridge, and a rather neglected bed of ballast remained where the other track had been removed.
SOMEONE had dug a hole right in the middle of the abandoned path, and removed several underlying bricks. Closer inspection showed that the entire bridge had been ‘paved’ with these bricks, prior to ballasting.
The bricks were a light yellow in color, reminding me of refractory brick.
Is that in fact what those bricks were? A fire shield just to protect the steel in case of fire/wreck?
I know the Lucien cutoff - the original timber trestle across the Great Salt Lake - was constructed with a solid deck (redwood) and then ballasted. An acquaintance has been salvaging that trestle for years for both the redwood and the pine timbers.
dd
PS - thanks MC for the hands on information. I see the MOW crews working the transistion between the open deck bridge across the Colorado River here in Austin and the fill that leads to the Austin Amtrak station all the time.
I believe that closed deck, ballasted bridges need a waterproof layer on the deck - so that rainwater drains away where it’s supposed to, and doesn’t seep into and damage the bridge structure - so maybe that’s what the brick layer is for ?
Tony
…Is there any so called “standard” of tonnage capacity of a main line railroad bridge for any given section on that bridge that the “industry” would build to…?
Thanks Mark., since you mentioned concrete, there was one addiotional “curio” about this bridge that made me ponder fire proofing. At least because I’ve never seen a bridge built quite this way.
If you will please forgive the crudness of the following sketch, depicting a cross section of the bridge span (to the best I can determine):
You see the side I beams, the lateral boxbeams spanning between the i beams at the base of the road bed, the layer of yellow brick beneath the two tracks as originally built, AND the added curio of the poured in place concrete “filler” on either side (drawn in dark grey) that in essence fills the inside web of the spanning I-beams, tooled to a neat radius near the bottom, almost as a ‘curb’ of sorts.
Seeing this cement layering on the inside of the I-beam, filling the entire inside web, was what made me ponder the possibility that the brick might be some form of fire shield. Since i can’t imagine what other useful purpose the concrete might serve? (the yellow brick base might cover the entire bed, even under the concrete sides, since from visual surface inspection, I have no way of knowing.
Anyway, I’m not being ornery, just curious, since if at the time of construction they were willing to pour in place the rather formid
All good points you make. And specific to the comment I copied above, the “stiffenening” potential you mention was an idea that dawned on me only after I started to prepare the rough sketch… I have found no builders plate but the vintage appears to be late 40’s or 50’s, just as steel reinforced concrete started to become popular for most non residential types of construction, so maybe I’ve got a transitional design here?
A tweener spaning between the engineers love affair with steel and that of reinforced concrete, perhaps?
No telling how much re-bar is in those side panels…
What is odd is to see that curved contour exposed above ground… it looks more like something that would be at home in an old subway.‘tube’.
Perhaps “love affair” was the wrong choice of words.
“First preferance” may have been a better choice
As in the Depresson era and earlier preferance for riveted steel framework ( Empire state building , and the sort) versus the poured slipform re-bar and cement style of construction. that became widely popular after WWII?
When it comes to change people get very emotional. I remember a C&NW grain rate guy objecting to significant abandoments in IA and MN because it would destroy the rate structure. He had devoted his life to that rate structure for 40 years.
Changing the ties on a ballast deck bridge is a walk in the park compared to changing the ties on an open deck structures. These open deck ties are machined on the bottom specifically for the bridge being redecked and must be custom ordered. There is a lead time for this and so if the ties on an open deck bridge are damaged due to a derailment or fire, the bridge may be out of service or slow ordered for months at a time. The the ties on a ballast deck structure are common hard wood ties just like the ties offf of the structure and commonly available from the railroad’s own stockpiles.
Replacing the ties on an open deck require special fittings, special tools and specially trained employees. Replacing the ties on a ballast deck bridge can be done by the normal tie and surfacing gangs using the mechanized equipment they are familiar with.
Yes, ballast deck bridges are quite a savings for the railroad and you commonly see open deck bridges changed to the ballast deck style if it is technologically feasible.
Is it the bridge over the St. Mary’s ?
http://terraserver.microsoft.com/image.aspx?T=4&S=8&Z=16&X=13087&Y=90965&W
Wow, the interest you (and the consultant) have put into this, surprises me, in a flattering sort if way, (if that makes any sense)
Yes, the general layout and scale of my crude sketch are reasonably accurate, TO the extent that memory can serve somone who did not bother to write down dimensions in the field.
This is a bridge that I’ve gone under in a boat for years, and driven by in a car for years,…and not until this year did i ever bother to physically walk up and across that dern thing, and yes I was a tad surprised at what I found.
Pictures? Yeah, it sounds like those might be in order, but it’s going to take me a while to get back out there and get pics, get them developed, etc so this will probably come back as a “hey Mark remember when?” thing in a few weeks, but in the mean time if it helps at all the bridge in question is at 41.0714 degrees north lattitude, 85.1623 degrees west longitude , according to topozone
I’ll see what I can do for pictures
Bingo, that is the one,…albeit from that angle the sides look much shorter than they do in person.
If you look at the inside edge of the bridge that is nearer to the top of the photo, you can see the lighter color strip that is the cement “filler”…
Mudchichen, what is Cooper-65 and Cooper-80?
Dogboy:
Coopers E-65 and E-80 ratings is the way railroad bridges have been designed since 1894 to withstand the impact loading as you roll a steam engine across the bridge. In simple terms, the E- signified the steam engine and the numeral was the weight in kips/1000 Lb. increments on the drivers (big wheels) on the locomotive. IE - E80 has 80000 Lb loading on the driving wheels of the locomotive . Without dragging out shear and moment diagrams, the bridge engineer drags out the old steam engine wheel arrangements to create the worst possible combination of load factors to affect the bridge structure under severe loading. Bridges of the first half of the 20th century were generally E-50 to E-65 back in the days of the 40 foot boxcar.The ratings now are meant to exceed Cooper’s E-80. Railroads can get very creative in finding ways to stiffen up the bridges for heavier loading. As an example, Santa Fe beefed up their timber bridges by adding two timber piles at each bent (pier) and then adding 14-20 pieces of T-rail (Scrap rail, 14 ft or 28 ft long to match span lengths) under the deck (Open or ballast deck) to handle the ever increasing loads. BN did other things and now the new BNSF is having quite an adventure mixing and matching standard bridge designs.